The use of cathodic deposition as an electrolytic technique for the extraction of conductive metals from a solution has long been known. Such techniques are utilized in various industries, including the photographic industry for recovery of silver from photographic processing solutions. Many electrochemical cells and methods are described in the literature, but have not been commercialized because of various disadvantages, particularly with respect to metal recovery from dilute solutions.
It is well known that efficiency in electrochemical processes, such as electrolysis, electroplating, electrowinning, electroorganic synthesis and waste recovery of metals from dilute solutions requires the use of cells with very high mass transfer characteristics or high surface area electrodes. Electrodes have been constructed with ridges or convolutions, or roughened in some manner, to increase the surface area. More recently, carbon fibers have been used as electrodes because they provide higher surface area.
By far, the most common types of electrolytic cells used to recover silver from photographic processing solutions comprise planar cathode cells. Most planar cells operate by passing the solutions across the cathode in a tangential manner, or by rotating the cathode to enhance the tangential flow with turbulence at the electrode boundary layer. Due to the relatively low surface area of planar cathodes, the performance of these cells remains insufficient for every silver recovery need in the photographic industry. It is also known to use mesh electrodes, toughened electrodes, metal and carbon foams and other porous electrodes. Porous electrodes substantially improve the mass transport characteristics within an electrochemical cell. However, this technology has not been embraced in the photographic industry because of various shortcomings.
Porous fibrous carbon or graphite cathodes are described for use in metal recovery from dilute solutions in U.S. Pat. No. 5,690,806 (Sunderland et al). Electrolytic cells containing such materials provide increased surface area and substantially increase the mass transport in a cell, but they are disadvantageous because of limitations of capacity, maintenance and efficiency. The use of a porous cathode support results in poor control of the required flow characteristics that are necessary for desired recovered metal crystal morphology. Uneven flow within the cell also leads to the formation of undesirable water-insoluble by-products, such as silver sulfide, and to metal deposition on the cathode in a dendritic fashion. This then leads to shortened cathode life (shorting out when dendrites on the cathode contact the anode), and to the collection of broken dendrites and other metallic debris at the bottom of the cell.
The Sunderland et al cell also relies predominantly upon a carbon felt to achieve a uniform current distribution along the cathode. This is readily achieved early in cell usage, but is diminished with time as the metal grows on the cathode. Fluid flow is thus a more important consideration for useful cathode life and uniform metal deposition.
Another concern with the described Sunderland et al cell is that it appears to be best used at fairly low metal concentrations, that is below 50 mg/l. Higher concentrations are said to be accommodated but would require more frequent changes in the cathode. This is not practical for many uses of such cells, especially in the photographic processing industry where silver waste levels may reach as much as 15-20 g/l.
Despite the considerable technology disclosed in the art and commercially available for electrochemical processes, there remains a considerable gap between the existing technology and the increasingly rigorous demands placed upon various industries including the photographic industry, for metal recovery.
For example, it is often necessary to use multiple stages of reclamation to achieve the necessary very low levels of metals in dischargable wastewaters. For silver recovery, the process typically begins with electrolytic desilvering to reach silver concentrations in the range of 100 to 500 ppm.
Thus, there remains a need for cost-effective electrochemical cells allowing for recovering metals at either higher or low concentrations (single digit parts per million), with rapid recovery rates (g/min instead of g/hr), high metal loading capacity, simplicity in design and use, easy removal of deposited metal, and compact equipment design. None of the known technologies satisfies all of these needs simultaneously.